in Arizona?
Figure 2. Adult harvest mite.
Figure 1. Larval harvest mite or chigger.
University of Nebraska, Department of Entomology
Illustrated by Joel Floyd in Venomous Animals
of Arizona, Arizona Cooperative Extension
Chiggers
Kim McReynolds, Area Extension Agent, Natural Resources, University of Arizona Cooperative Extension,
Cochise, Graham and Greenlee Counties
Many people will tell you that chiggers do not occur in Arizona. I
have first hand experience as the recipient of numerous itching welts
that they were responsible for producing right here in Arizona. One
book that I read states that chiggers seem to occur only in the southern
part of the state. While I have encountered them in several mountain
ranges of southeastern Arizona, I have also had chigger bites in
Pinetop and along the Verde Rim.
Chiggers are the larval stage of the harvest mite in the Trombicula
family (Fig. 1). Closely related to ticks and spiders, adult harvest mites
have eight legs (Fig. 2). They are usually red in color and can be easily
seen walking across the soil surface. Adult harvest mites lay eggs in
the soil during the spring, from which the larval stage (chiggers) hatch
later in the year. My Arizona encounters with chiggers have typically
been during late summer or early fall when vegetation growth is hitting
its peak production for the year. At this stage of development, the
chiggers have only six legs and are about 1/50th of an inch in size.
Hungry for their first meal, chiggers climb to the top of grass or other
higher points and wait for animals to pass by and jump onto them.
Humans are not primary hosts for chiggers. They prefer rodents, birds
and other vertebrate animals.
Once on a human, chiggers tend to migrate to areas where clothes
are confined. Bites tend to be found around the ankles, behind the
knees, around the waistband, and with less frequency, the armpits.
The chiggers clasp onto the skin with specialized mouth parts, injecting
saliva filled with enzymes that break down skin tissue. The skin cell
contents are what the chiggers feed on, not blood as some people
think. Another myth that I remember from childhood is that chiggers
burrow under your skin. They actually attach themselves near a skin
pore or hair follicle. When the chigger injects the enzyme, our bodies
react by hardening the cells around the saliva-injected cell. This makes
a tube that the chigger uses to drink the cell contents. This tube is
called a stylostome, which is actually the thing that becomes irritated
and red and produces an intense itch.
Chiggers that attach themselves to humans probably never finish
their meal. They will most likely be brushed, scratched, or showered off
within a couple of hours. On other host animals, once the chigger has
had its fill (3-4 days), it will drop from the host. It will then proceed to the
nymph stage (where it develops eight legs), and later become an adult.
At these two later stages, the mites are harmless to humans.
It is nearly impossible in Arizona to recognize and avoid chigger
infested areas. If you are going to be spending some time in areas
of heavy vegetation (including open grasslands) some precautions
can be taken. Wearing long pants, long sleeve shirts and high top
boots or shoes will help. Make sure that they fit snuggly, but not tight
as this may become a possible feeding site. Tucking pants into boots
can also be helpful. Applying insect repellent on your skin and to your
clothing around your ankles, wrists, neckline and waist will increase
your chance of avoiding bites. Be sure to reapply repellents as often as
necessary since the effects of many will only last for a couple of hours.
If you happen to come home with chigger bites, expect the itching to
last for a week to ten days. There are a few things you can do for some
relief. Take a hot, soapy shower or bath as soon as you return home.
This will remove any chiggers that may still be hanging on. Applying
over-the-counter anti-itch lotions and creams will give some temporary
relief. Another popular myth associated with chigger bites is that
applying nail polish to the bite will suffocate borrowing chiggers. Since
chiggers don’t borrow into the skin, there is no value in this particular
treatment. It is important to realize that the source of the itching is
the stylostome, and until your body completely breaks that down and
absorbs it, it will continue to itch. While scratching seems to give some
short-term relief, it only further irritates the stylostome and can lead to
secondary infections.
Summer 2008
17